Wind turbines comprise pitch drives for rotating the rotor blades longitudinally in order to control the power extracted by the rotor and to mitigate the loads suffered by the structure as wind speed changes. By continuously adjusting the blade pitch angle, the aerodynamic angle of attack is adapted so that the influence of the wind on the blades can be increased or decreased. Each blade may have its independent pitch drive. Alternatively, a common pitch drive may be provided for a plurality of blades.
Blade feathering, consisting of turning the blades about 90 degrees about their longitudinal axis, is performed during emergency shutdowns, or whenever the wind speed exceeds the maximum rated speed, so that aerodynamic braking is generated to stop the wind turbine. Moreover, during maintenance of wind turbines, the blades are usually feathered to reduce unwanted rotational torque in the event of wind gusts.
A wind turbine pitch drive may comprise an electric motor which is powered by the electric grid via a power electronic converter. The electric motor may be an AC motor, or alternatively a DC motor. In other cases, a hydraulic pitch motor may be used. Depending on the motor type, the electronic converter may include an AC to DC power converter (rectifier), a DC link (capacitor bank), and a DC to AC power converter (inverter).
The pitch motors drive an actuator (e.g. a pinion or a hydraulic piston) which rotates the blade.
A back-up energy storage unit comprising a battery or a capacitor may be coupled to the electronic converter DC-link circuit by diodes for buffering during voltage fluctuations or transient voltage dips. The energy required can thus be drawn from the storage units. Furthermore, these energy storage units ensure the reliable functioning of the blade pitch drive in the event of a complete loss of power from the electric grid or when used during maintenance operations.
It is known that back-up energy storage units within modern wind turbine pitch systems may comprise one or more ultra-capacitors. Ultra-capacitors are particularly suited for remote and offshore wind power applications because of their high reliability, their efficiency, their easy monitoring and their long operating life (i.e. high number of charge/discharge cycles). Batteries, on the other hand, require ongoing measurement of their state of health and state of charge to avoid costly repairs or unsafe operating conditions.
However, whilst a traditional electrochemical battery releases its energy through processes that limit discharge currents; this is not the case with ultracapacitors. Thus, when the ultracapacitors are fully charged and the voltage at the DC-link to which they are connected is at or close to zero volts (e.g. because grid voltage is zero or no connection with the electrical grid exists during maintenance operations, so the capacitors at the DC-link are discharged), the ultracapacitors discharge from its initial charge state to zero voltage in a very short time, thus resulting in very high in-rush currents which can damage the diodes, transistors and other electronic components within the electronic converter.
This problem is currently being addressed by manually pre-discharging the energy storage units, which is a time and resource consuming task. Furthermore, as ultracapacitors are especially used in offshore installations, the frequency of manual operations is further required to be kept at a minimum due to very high costs related to such activities.
Therefore, there is a need for systems for wind turbine pitch drive which protect the connected electrical components and methods for protection during grid power outage or in maintenance mode.